Maleimides and citraconimides as curing agents for vulcanizable rubbers



. U ited States Patent Q 3,153,014 MALE to AND crrnacornns As CURING AG NTS FOR VULCANIZABLE nunnnns mixture with a curing agent which is monomaleimide,

monocitraconimide, or a Nsubstituted'derivative of either of these parent compounds having but one imido N atom.

Preferably, the vulcanization is accelerated with a thiazole-type accelerator, or, in some cases, with a free-radical generator such as an organic peroxide or an aliphatic azo compound. When the rubber is synthetic, the desirable physical characteristics of the vulcanizate are enhanced by employing a filler. This application is a division of Serial No. 811,532, filed May 7, 1959, now abandoned.

Most of the N-substituted monomaleimides which are used in this invention can be made by the method disclosed by Searle in US. Patent No. 2,444,536, issued July 6, 1948, and by Arnold and Searle in US. Patent No. 2,462,835, issued Marchl, 1949. They described many of the monomaleimides named in the present working examples. The following monornaleimides, believed to be 7 new, also can be made by their'process.

Product: Melting point, C. N-4-anisylmaleimide 151-4.

N-4-acetylphenylmaleimide 157.560.0. .N-2,4-xylylmaleimide i.. 74.57.0. N-2,6-xylylmaleimide 108.09.2.

" ,N-2-chlorophenylmaleirnide 57.S9.0.

N-3-chlorophenylmaleimide 87.0-95. N-isopropylmaleimide About 23 (B.P. N-4-t-butylpheny1maleirnide 70-3/5 mm.).

The N-alkylmaleimides also can be made merely by heating the corresponding N-alkylmaleamic acid in vacuo.

Several N-alkylmaleimides are described by Beilstein (4th ed.), vol. 21.

The monocitraconimides used in this invention are made by the same methods by which the corresponding maleimides are made. Many of the citraconimides are described by Beilstein, loc. cit. Some typical monocitraconimides are citraconimide itself, N-isopropylcitraconimide, N-phenylcitraconirnide, the N-(chlorophenyDcitraconimides, N-(2,4-dinitrophenyl)citraconimide, the N- tolylcitraconimides, the N-naphthylcitraconimides, and the N-(methoxyphenyl) citraconimides.

Monomaleimide, monocitraconimide, and the N-substituted derivatives thereof having butone imido N atom can be used according to the invention in widely varying amounts. .However, we prefer to use from about 0.25 to'about 6.0 partsof the curing agent per 100 parts of the rubber. It is understood that the term curing agent includes both a single compound and a mixture of two or more compounds as defined above, and the term rubber includes both a single rubber and a mixture oftwo or niore rubbers to be defined below.

7 3,153,014 Patented Oct. 13, 1964 "ice The rubbers which are operable in this invention are the natural and the synthetic rubbers which have high olefinic unsaturationahd which are conventionally vulcanized with sulfur. 1 Such synthetic rubbers are the homopolymers of aliphaticv conjugated diolefin hydrocarbons and copolymers of such diolefins with monoolefinic compounds copolymerizable therewith by emulsion polymerization methods. Such monoolefins include styrene; alpha-methylstyrene; p-methylstyrene; alpha, p-dimethylstyrene; acrylic and methacrylic nitriles, amides, acids and esters; vinyl pyridines; fumaric esters; methylenemalonic esters; vinylidene chloride; methyl vinyl ketone; and methyl isopropenyl. ketone. Mixtures of such monoolefinic compounds can also be copolymerized with the diolefin. The term high olefinic unsaturation here connotes an amount of unsaturation on the order of that occuring in Hevea rubber. The copolymers must contain copolymerized therein at least about 35% of the diolefin hydrocarbon. The butyl rubbers, which are elastomers made by an ionic polymerization process, from a major amount of an isoolefin and a minor amount of a conjugated diolefin hydrocarbon in an oragnic solvent, are not curable with maleirnides, and are excluded from the scope of the invention.

The satisfactory operation of this invention does not require a filler when the rubber is natural or Hevea rubben; In the case of a single synthetic rubber or a mixture of two or more synthetic rubbers, it is important to employ a filler, the minimum quantity thereof being well known to those skilled in the art of rubber compounding. Generally, we use at least 10 parts by weight of filler per parts by weight of rubber, although this minimum is not critical and can be reduced in most cases without destroying the desirable physical characteristics of the vulcanizate.

The preferred fillers are the carbon blacks and the hydratecl silicas. However, other fillers conventionally used in the rubber industry also are operable in our invention. Such fillers are titanium dioxide, clay, whiting, etc. Of course, the physical properties of the vulcanizates will vary considerably depending on the kind of filler used, as also is well known to anyone skilled in rubber compounding. So far as vulcanization is concerned, the maximum amount of filler is not critical. Those skilled in the art will understand that the practical maximum is that figureat which the physical properties of the vulcanizate begin to fall ofi objectionably.

We prefer to use butadiene-l,3, as the conjugated diolefin hydrocarbon in the synthetic homopolymers and copolymers, but other conjugated diolefin hydrocarbons which contain as many as six carbon atoms may be used, e.g., isoprene, piperylene, and 2,3-dimethylbutadiene.

The styrene/butadiene copolymer rubbers used in our invention are conventionally termed SBRJ? in accordance with A.S.T.M. recommendations.

Among the acrylic-type monomerswhich may be used in making copolymer rubbers to be cured by the method of the invention are acrylonitrile; methacrylonitrile, acrylic acid and its alkyl esters, e.g., methyl acrylate, ethylacrylate, butyl acrylate, and Z-ethylhexyl acrylate; methacrylic acid and its alkyl esters; acrylamide, N- rnonoalkylarnides, N-monoaralkylacrylamides, N-diaralkylacrylamides. The most important of these monomers is acrylonitrile, the corresponding diolefinzacrylonitrite elastomers being conventionallygnow called NBR,

sold commercially under the names Paracril, Hycar, etc.

Typical vinylpyridines are 2-vinylpyridine, 3-vinylpyridine, 4-vinylpyridine, 2-methyl-5-vinylpyridine, S-ethyl- 2-vinylpyridine, 2-methyl-6-vinylpyridine, 2-ethyl-4-vinylpyridine, etc.

Among the fumarates are the symmetrical and unsymmetrical alkyl esters of fumaric acid, e.g., diethyl fumarate, ethyl methyl fumarate.

Among the methylenemalonic esters are the esters with an alkanol, more specifically diethyl methylenemalonate, diisopropyl methylenemalonate, di-n-butyl methylene malonate, diisobutyl methylenemalonate, dimethyl methylenemalonate, etc. Unsymmetrical esters also can be used.

The synthetic rubber, filler, and curing agent of the present invention, together with any other desired materials such as accelerators of the type described below, plasticizers, antioxidants, and other conventional rubber compounding ingredients, are intimately mixed in any convenient manner used in the rubber industry, e.g., on a rubber mill or in an internal mixer.

The temperature of mixing can vary between 160 and 275 F. depending upon the amount and kind of filler, and the type of mixing equipment. The compounded rubber is then converted to any desired shape and size, and is vulcanized at temperatures from 200 F. to 400 F. for from 2 minutes'to 8 hours depending on the size and shape of the article being cured. Cures may be made in any well-known Way as in a mold under pressure or in an open container in an oven.

A further feature of our invention comprises the addition of general purpose thiazole-type accelerators, such as Z-mercaptobenzothiazole, 2,2-dibenzothiazyl disulfide, and the benzothiazolesulfenamides, such as N-oxydiethylene 2-benzothiazolesulfenarnide and N-cyclohexyl-2- benzothiazolesulfenamide. These vulcanization accelerators, when added to the rubbers, vulcanizing agent and filler mixtures, greatly increase the rate of vulcanization induced by the monomaleimides and monocitraconimides. In many cases, the time of vulcanization can be reduced to one-half or even less by this acceleration. Alternatively, at the operators convenience, the time can be kept constant and the temperature decreased below that used to obtain an equivalent cure without an accelerator.

The amount of accelerator can usefully range from 0.25 part to 2.0 or more parts.

The vulcanization procedure of the present invention has many advantages over previously known vulcanizing processes. Among these are the following:

(1) The rubber stocks vulcanized by the new process of our invention have a far better resistance to oxidative aging than does rubber vulcanized with sulfur. For example, when aged in air at 100 C., the new stocks of our invention deteriorate much less rapidly than a standard sulfur stock. Thus, these new stocks are especially useful in products which must operate for considerable periods of time at high temperatures. curing bags for tires, rubber motor mountings, steam hose, gaskets and belts for hot machinery, conveyor belts for moving hot materials, flexible hot air ducts, hot water bottles, etc.

(2) The new stocks of the present invention may be used in contact With metals such as copper, silver, etc. which are tarnished by stocks vulcanized by sulfur. The advantages of using these new non-sulfur vulcanizates for rubber-metal objects, e.g., composite rubber and metallic cloth or fabric articles, rubber-insulated wire, headlights, silverware, copperware, etc. are obvious.

(3) The compounded but unvulcanized stocks made in accordance with the principles of our invention can be processed at higher temperatures without scorching than can stocks containing sulfur as the vulcanizing agent. This is particularly advantageous when mixing compounded rubber in largebatches in Banbury mixers, usually Such products are tires,

operated at quite high temperatures, and when articles by injection molding.

The following examples illustrate the invention. All parts are by weight.

shaping Example 1 Example 1 illustrates the gist of the invention, i.e., the practical cure of the highly unsaturated rubber-like commercial SBR with monomaleimides. A masterbatch 1n the proportion of parts of commercial SBR comprising butadiene and styrene in the ratio 76:24 and made at 5 C., and 50 parts of carbon black was made in a Banbury internal mixer. Portions of the masterbatch were mixed on a rubber mill with various maleimides to form the stocks below. The stocks were cured as shown in molds under pressure. .After the cured stocks had returned to room temperature they were tested conventionally to determine extent of cure. The tensile strength and elongation were measured on a Scott tensile tester, the modulus on an autographie stress-strain tester.

Stock 1 2 Masterbatch 150 150 Maleimide 2 N-phenylmaleimide 2 PHYSICAL PROPERTIES A. Cured 60 min. at 166 0.:

Tensile strength (p.s.i.) 250 430 Elongation (perceut) 630 620 200% modulus (p.s.i.) r

300% modulus (p.s.i.) 170 220 B. Cured 60 min. at 0.:

Tensile strength (p.s.i.) 800 1,090

Elongation (percent) 670 600 200% modulus (p.s.i. 210 270 300% modulus (p.s.i.). 330 440 The above example shows that monomaleimides cure SBR. It also shows that modulus and tensile strength properties are improved markedly by increasing the temperature of vulcanization.

Example 2 Example 2 demonstrates that it is possible to accelerate the monomaleimide cure of a highly unsaturated rubber using a thiazole-type accelerator. The following stocks were mixed, cured, and tested as in Example 1.

Stock 3 4 Masterbatch (see Ex. 1)" Maleimide N-phenylmaleimide. 2-mereaptobenzothiazole rnrsroar. raornnrrns A. Cured 60 min. at 166 0.:

Tensile Strength (p.s.i.) 2, 280 2, 500 Elongation (percent) 600 520 200% modulus (p.s.i.) 480 460 300% modulus (p.s.i.) 710 000 B. Cured 60 min. at 195 0.:

Tensile strength (p.s.i.) 2, 310 2, 250 Elongation (percent)- 460 380 200% modulus (p.s.i.) 540 700 300% modulus (p.s.i.) 980 1, 280

(described in Example 1), 50 parts of carbon black, and

5 parts of a hydrocarbon plasticizing oil was made in a Banbury. Portions'of the Masterbatch were mixed on a mill with the materials shown individually to form the stocks shown below. They were cured at 153 C. for

the times shown below and tested as in Example 1.

Stock s 5 Masterbateh 155 155 N-phenylmaleimide 4 4 2,2'-dibenz0thlazyl disulfide 1 2 PHYSICAL rnornarms Tensile strength (p.s.i.)

Time of cure (min):

180 2, 970 2, 940 Elongation (percent) Time of cure (mm):

30 640 500 45-.- 570 480 60..- 530 450 90. 520 570 180 510 530 300% modulus (p.s.i.)-- V Time of cure (min,):

This example shows by comparison with Example 1, that 2,2'-dibenzothiazyl disulfide' is a powerful accelerator of cure for the monomaleimides.

Example 4 Stock Masterbatch N-phenylmaleimide (0.023 mole) N-ethylrnaleimide (0.023 mole) N-isopropylmaleimide (0.023 mole) 2,2-dibenzothiazyl disulfide PHYSICAL PROPERTIES Tensile strength (p.s.i.) Time of cure (min):

2, 240 1, 280 1, 090 2, 190 2, 230 1, 960 90 2, 330 2, 140 2, 350 Elor ilgationpercentl imeo cure min. 2 22 -5 630 770 790 45.. 430 710 600 90 440 600 670 3002 1 modulus (p.

eo cure in. m 700 325 325 The above example shows that N-alkylmaleimides,

- when accelerated with a conventional thiazole-type accelerator like 2,2-dibenzothiazyl disulfide, will cure highly unsaturated rubbers such as SBR. On a molar basis, the N-alkylmaleimides appear to be slower curing agents than the N-arylmaleimides.

Example 5 Example 5 demonstrates that the successful operation of this inventionis not limited to the inclusion of a single filler like carbon black.

The following stock was mixed ona mill, cured at 145 C., and tested as shown in Example 1 '75 Stock-.- 10

SEE Titanium dioxide 40 Ola 70 Hydrocarbon plastieizing oil 5 N-phenylmnlpimido 4 2,2-dibenzothiazyl disulfide 2 PHYSICAL PROIERTIES Tensile strength (p.s.1.)-

Time of cure (min):

30 500 4% 690 00- 830 Elongation (pereent) Time of cure (min):

20 1, 220 45 1,130 00 1, 050 300% modulus (p.s.i.)-

Time of cure (miu.):

an 45 170 an 230 The above example shows that fillers other than carbon black, for example clays and/or pigments, can be used in this invention.

Example 6 Example 6 further demonstrates the operation of this invention using substituted N-arylmaleirnides as curing agents.

A masterbatch in the proportion of 100 parts of SBR, 50 parts of carbon black, and 8 parts of. hydrocarbon plasticizing oil, was made in a Banbury; Portions of the masterbatch were mixed on a mill with equimolar amounts of various maleimides to form the stocks shown below. The stocks were cured at C. and tested as in Example 1.

Stock 11 12 13 Masterbatch 158 158 158 N-phenylmaleimide. N-p-anisylmaleimide. 4. 7 NA-aoetyliphenylmals-imi 4. 98 2,2-dibenzothiazyl disulfide 2 2 2 PHYSICAL PROPERTIES Tensile strength (p,s.i.)' Time of Cure (min):

30 l, 970 1,730 1, 590 2, 190 2, 100 1, 780 2,180 2, 230 1, 780 120 2, 200 2, 050 1, 890 Elongation (Percent)- Time of Cure (min):

30 540 510 480 45..- 460 480 420 60.- 430 430 390 120 380 360 380 300% modulus (p.s.1.)

Time of Cure (min):

30 810 800 850 1, 130 1, 190 1, V 1, 340 1, 360 1, 320 120 .r 1, 650 1, 550 1,480 Torsional hysteresis (138 C.) measured per the method of the Gerke et al. Patent 2,118,601-

Time of Cure (min.)*

This example has shown that the phenyl group of the N-arylmaleimide curing agent can have substituents on it.

Example 7 Example 7 demonstrates the operation of this invention with a wide variety of substituted N-arylmaleimides in Stock 14 Masterbateh Zine oxide Stearie acid 2-mercaptobeuzothiazole Diphenylguanidine Sulfur N-Z-nitrophenylmaleimide1 N-3-nitrophenylmaleimide. N-i-nitrophenylmaleimide. N-2-t0lylmaleimide N -3-t0lylmaleimide N-4toly1maleimide N-2,4-xyl ylmaleimide N-2,6-xyly1maleirnid e. N-4-phenethylmaleimide N-2-ehlorophenylrnaleimide N-3-cli1oropheny1maleimide. N-l-naphthylmaleimider 2,2-dibenzthiazyl disulfide 8 Example 8 All of the preceding examples, 1 to 7, illustrate the operation of this invention for the cure of SBR rubber. The following examples'illustrate that other highly unsaturated rubbers are cured by the method of this invention. In particular, Example 8 illustrates the cure of natural rubber with N-arylmaleimides accelerated by Time of aging (days) 0 4 0 4 0 4 0 4 0 4 0 4 PROPERTY Tensile (p.s.i.)

Cure time (min):

90 2,030 2,500 2,010 2,130 2,110 2,080 ,670 1,030 2,610 2,460 2,680 2,000 180 .1 2,480 2,650 2,110 2,200 2,270 2,100 1,730 1,920 2,000 2,570 2,780 2,620 Elongation (percent) Cure time (min):

90 1 390 250 590 520 620 500 630 510 490 480 440 400 180 350 300 560 480 570 470 570 510 470 450 430 370 300% mod. (p.s.i.)-

Cure time (min):

Stock 21 22 23 24 25 26 Masterb atch Zine oxide- Stearie acid 2-mercaptoben zothiazole.

Diphenylguanidine Sulfur N -2-nitrophenylmaleimide. N-3'nitropheny1maleimida N -4-nitropl1enylmaleixnide N-2-tolylmaleirnide N-3-tolylmaleimide N -4-tolylmaleimide. N-2,4-xylylmaleimide N-2,6 xylylmaleimide. N-4-phenethylmaleimide N-2-chloropheuylmaleimi N-3-ehlorophenylmaleimide N=1-naphthylmaleimide 2,2-dibenzothiazyl disulfi Cure time (min):

Time of aging (days) .1 0 4 0 4 0 4 0 4 0 4 0 4 0 4 PROPERTY Tensile (p.s.i.)--

Cure time (min):

90 2, 680 2, 340 2, 680 2, 490 2, 370 2, 2, 730 2, 490 2, 820 2, 400 2, 770 2, 300 3, 220 2, 530 2, 0 2, 320 2, 880 2, 500 2, 610- 2, 290 2-, 880 2, 550 2, 870 2, 470 2, 820 2,430 2, 790 2, 530 Elongation (peree Example 7 shows many N-arylmaleimides which are operable in this invention. The heat aging, i.e., resistance to stiffening, as shown by the modulus, of stocks 15-26 which illustrate this invention, is very much better than that of stock 14 which illustrates conventional sulfur cure.

2,2-dibenzothiazyl disulfide.

The following stocks were mixed, cured at 145 C, and tested as in Example 1, except that the masterbatch consisted of Hevea rubber (smoked sheet) and carbon black in the proportion of 100:50. i

2.2'-dibenzothiazyl disulfide N-phenylmaleimide (0.023 m N-p-tolylmaleimide (0.023 moles) N-panisylmaleimide (0.023 moles) N-eacetylphenylmaleimide (0.023

moles) PHYSICAL PROPERTIES Tensile strength (p.s.i.)

Tim; of cure (min):

2, 410 2, 470 2, 400 2, 570 60-. 2, 440 2, 540 2, 770 2, 610 120 2, 480 2, 610 2, 730 2, 710 Elongation (percent) Time of cure (min.):

570 570 500 570 540 530 570 530 120 520 520 530 520 300% modulus (p.s.i.)

Time of cure (min):

e Condensation product of diphenylamine and acetone.

The above example shows that carbon black reinforced natural rubber can be satisfactorily cured by the method of this invention using a variety of N-arylmaleimides.

Example 9 Example 9 illustrates the operation of this invention for the cure of a carbon black reinforced vinylpyridine/butadiene rubber with a variety of N-arylmaleimides.

A masterbatch in the proportion of 100 parts of 2- methyl-S-vinylpyridine:butadiene rubber (25:75), 50 parts of carbon black, and parts of hydrocarbon plasticizing oil was made in a Banbury. Portions of the masterbatch were mixed on the mill with the materials shown below to form stocks which were cured at 145 C.

and tested as in Example 1.

Stock 31 32 33 34 Masterbatch 155 155 155 155 2,2 -dibenzothiazyl disulfide 2.0 2. 0 2. 0 2. 0 N-phenylmaleirm'de N-p-tolylmaleimide N-p-anisylmaleimide N-Z-chlorophenylmaleimi 4. 7

YHYSICAL PROPERTIES Tensile strength -(p.s.i.)

Time of cure (min.):

22 1, 540 1, 710 1, 640 1, 060 45- 1,680 1, 770 1,690 1, 200 90 1, 850 1, 990 1, 850 1, 380 Elongation (perc Time of cure (min):

810 800 810 860 810 740 750 850 750 700 740 820 300% mod (p.s.i.)

Time of cure (min):

The above example shows that a vinylpyridine rubber can be cured by the method of this invention. I

Example 7 B I-Ii-Sil #233, Wet precipitated silica filler marketed by Columbia Southern Chemical Corp.

The above example shows that other non-black reinforcing fillers such as the wet precipitated silicas can be used in this invention.

Example 11 Example 11 demonstrates that it is possible to use other types of general purpose thiazole accelerators to accelerate the monomaleimide cure or highly unsaturated rubbers.

A masterbatch in the proportion of 100 parts of SBR (described in Example 1), 50 parts of carbon black, and 8 parts of hydrocarbon plasticizing oil was made in Banbury mixer. Portions of the masterbatch were mixed on a mill with the materials shown individually to form the stocks shown below. They were cured at 153 C. for the time shown below and tested as in Example 1.

Stock 36 37 38 Masterbatch 158 158 158 N-Phenylmaleimide 4. 0 4. 0 4. 0 2,2-dibenzothiazyl disulfi 2 N-oxydiethylene-Z-benzothi csulien- PHYSICAL PROPERTIES Tensile strength (p.s.i.)

Time of cure (min):

22 2, 900 2, 650 2, 370 45 3,800 2, 940 2, 460 8,140 3,060 2, 570 Elongation (percent)- Time of cure (min):

. 22 540 680 7 10 45- 480 590 710 90 460 580 710 300% modulus (p.s.1.)

Time of cure (mm):

The above example demonstrates the accelerating effect of two other general purpose thiazole-type accelerators on the monomaleimide cure of a highly unsaturated rubber such as 5BR.

Example 12 Example 12 illustrates that a practical cure of highly unsaturated rubber-like commercial SBR can be obtained with monocitraconimides. A masterbatch was prepared as in Example 1.

Portions of this masterbatch were mixed on a rubber itself and N-phenylcitraconimide.

Stock 39 40 The stocks were cured under pressure and tested conventionally to determine the extent of cure. The physical characteristics, i.e., tensile strength, elongation, and modulus, were substantially similar to those obtained with stocks 1 and 2 of Example 1. These data show that SBR can be cured with monocitraconimides and that the physical properties of the rubber are improved by employing higher vulcanization temperatures.

Having thus described our invention, what we claim and desire to protect by Letters Patent is:

l. A process for vulcanizing highly unsaturated rubbers selected from the group consisting of homopolyrners of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, and heteropolymers of such diolefin hydrocarbons with copolymerizable rnonooleiinic com.- pounds, said heteropolymers containing at least 35% of said diolefin hydrocarbon copolymerized therein, com-' prising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprised of 100 parts by weight of the rubber, and from 0.25 to 6.0 parts by weight of a curing agent selected from the group consisting of maleimide, monocitraconimide, and the N-substituted derivatives thereof having but one imido N atom.

2. A process for vulcanizing highly unsaturated rubbers selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, and heteropolymers of such diolefin hydrocarbons with copolymerizable monooiefinic compounds, said heteropolymers containing at least 35% of said diolefin hydrocarbon copolymerized therein, comprising heating at a temperature of firom,200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprised of 100 parts by weight of the rubber, and from 0.25 to 6.0 parts by Weight of a curing agent selected from the group consisting of monomaleimide, monocitraconimide, N-alkyl and N-aryl monomaleimides, and N-alkyl and N-aryl monocitraconimides.

3. A process for vulcanizing highly unsaturated rubbers selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefinic compounds, said heteropolyrners containing at least 35% of said diolefin hydrocarbon copolymerized therein, comprising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprised of 100 parts by weight of the rubber, and from 0.25 to 6.0 parts by weight of maleirnide.

4. A process for vulcanizing highly unsaturated rubbers selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefinic cornpounds, said heteropolymers containing at least 35% of said diolefin hydrocarbon copolymerized therein, comprising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprised of 100' parts by weight of the rubber, and from 0.25 to 6.0 parts by Weight of N-phenylmaleimide.

5. A process for vulcanizing highly unsaturated rubbers selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, and heteropolymers of such diolefin said diolefin hydrocarbon copolymerized therein, comprising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprised of parts by weight of the rubber, and from 0.25 to 6.0 parts by Weight of N-ethylmaleimide.

6. A process for vulcanizing highly unsaturated rubbers selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefinic compounds, said heteropolymers containing at least 35% of said diolefin hydrocarbon copolymerized therein, comprising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprised of 100 parts by weight of the rubber, and from 0.25 to 6.0 parts by weight of N-3-chlorophenylmaleimide.

7. A process for vulcanizing highly unsaturated rubbers selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefinic compounds, said heteropolymers containing at least 35% of said diolefin hydrocarbon copolymerized therein, comprising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprised of 100 parts by Weight of the rubber, and from 0.25 to 6.0 parts by Weight of N-l-naphthylmaleimide.

8. A process for vulcanizing a rubber which is a homopolymer of an aliphatic conjugated diolefin hydrocarbon having up to 6 carbon atoms, comprising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprising 100 parts by weight of the rubber, and from 0.25 to 6.0 parts by weight of a curing agent selected from the group consisting of monomaleimide, monocitraconimide, N-alkyl and N-aryl monomaleimides, and N-alkyl and N-aryl monocitraconimides.

9. A process as in claim 8, wherein the unvulcanized rubbery reaction mass is admixed prior to heating with from 0.25 part to about 2.0 parts by weight of thiazole accelerator.

10. A process for vulcanizing a rubber which is a homopolymer of an aliphatic conjugated diolefin hydrocarbon having up to 6 carbon atoms, comprising heating at a temperature of from 200 F. to 400 F. for a period of from 2 minutes to 8 hours a mixture comprising 100 parts by weight of the rubber, from 0.25 to 6.0 parts by Weight of N-phenylmaleimide as a curing agent, and from 0.25 to 2.0 parts by weight of mercaptobenzothiazole as an accelerator.

11. A vulcanizate comprising the heat-reaction product of 100 parts of rubber selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefin compounds, said heteropolyrners containing at least 35% of said diolefin hydrocarbon copolymerized therein, and from 0.25 to 6.0 parts of a curing agent selected from the group consisting of maleimide, monocitraconimide and N-substituted derivatives thereof having but one imido N atom.

12. A process for vulcanizing a rubber which is a homopolymer of an aliphatic conjugated diolefin hydrocarbon having up to 6 carbon atoms, comprising heating at a temperature of from 200 F. to 400 F, for a period of from 2 minutes to 8 hoursv a mixture comprising 100 parts by weight of the rubber, from 0.25 to 6.0 parts by weight of N-phenylrnaleimide as a curing agent, and

hydrocarbons with copolymerizable monoolefinic compounds, said heteropolymers containing at least 35% of from 0.25 to 2.0 parts by weight of 2,2-dibenzothiazyl disulfide as an accelerator.

13. A vulcanizate comprising the heat-reaction product of 100 parts of rubber selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefinic compounds, said heteropolymers containing at least 35 of said diolefin hydrocarbon copolymerized therein, from 0.25 to 6.0 parts by weight of a curing agent which is a mono-imide compound selected from \the group consisting of monomaleimide, monocitraconimide, N-alkyl and N-aryl monomaleimides, and N-alkyl and N-aryl monocitraconimides, and from 0.25 to 2.0 parts of thiazole accelerator.

14. A vulcanizate comprising the heat-reaction product of 100 parts by weight of rubber selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms and heteropolyrners of such diolefin hydrocarbons with copolymerizable monoolefinic compounds, said heteropolymers containing at least 35 of said diolefin hydrocarbon copolymen'zed therein, and from 0.25 to 6.0 parts by weight of N-phenylmaleimide as a curing agent.

15. A vulcanizate comprising the heat-reaction product of 100 parts by weight of rubber selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefi-nic compounds, said 1 heteropolymers containing at least 35 of said diolefin hydrocarbon copolymerized therein, and from 0.25 to 6.0 parts by weight of N-ethylmaleimide as a curing agent.

16. A vulcanizate comprising the heat-reaction product of 100 parts by weight of rubber selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefinic compounds, said heteropolymers containing at least of said diolefin hydrocarbon copolymerized therein, and from 0.25 to 6.0 parts by weight of N-3-chlorophenylmaleimide as a curing agent.

17. A vulcanizate comprising the heat-reaction product of parts of rubber selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms, heteropolymers of such diolefin hydrocarbons w'th copolymerizable monoolefin compounds, said heteropolymers containing at least 35% of said diolefin hydrocarbon copolymerized therein, a filler, and from 0.25 to 6.0 parts of a curing agent selected from the group consisting of maleimide, monocitraconimide and N-substituted derivatives thereof having but one imido N atom.

18. A vulcanizate comprising the heat-reaction product of 100 parts by weight of rubber selected from the group consisting of homopolymers of aliphatic conjugated diolefin hydrocarbons having up to 6 carbon atoms and heteropolymers of such diolefin hydrocarbons with copolymerizable monoolefinic compounds, said heteropolymers containing at least 35% of said diolefin hydrocarbon copolymerized therein, and from 0.25 to 6.0 parts by weight of N-l-naphthylmaleimide as a curing agent.

References Cited in the file of this patent UNITED STATES PATENTS 2,477,015 Sturgis et a1. July 26, 1949 2,925,407 Goldberg Feb. 16, 1960 2,958,672 Goldberg Nov. 1, 1960 

1. A PROCESS FOR VULCANIZING HIGHLY UNSATURATED RUBBERS SELECTED FROM THE GROUP CONSISTING OF HOMOPOLYMERS OF ALIPHATIC CONJUGATED DIOLEFIN HYDROCARBONS HAVING UP TO 6 CARBON ATOMS, AND HETEROPOLYMERS OF SUCH DIOLEFIN HYDROCARBONS WITH COPOLYMERIZABLE MONOOLEFINIC COMPOUNDS, SAID HETEROPOLYMERS CONTAINING LAT LEAST 35% OF SAID DIOLEFIN HYDROCARBON COPOLYMERIZED THEREIN, COMPRISING HEATING AT A TEMPERATURE OF FROM 200*F. TO 400*F. FOR A PERIOD OF FROM 2 MINUTES TO 8 HOURS A MIXTURE COMPRISED OF 100 PARTS BY WEIGHT OF THE RUBBER, AND FROM 025 TO 6.0 PARTS BY WEIGHT OF A CURING AGENT SELECTED FROM THE GROUP CONSISTING OF MALEIMIDE, MONOCITRACONIMIDE, AND THE N-SUBSTITUTED DERIVATIVES THEREOF HAVING BUT ONE IMIDO N ATOM. 